This invention relates to a method and apparatus for transferring digital data between a mass memory, such as a magnetic disc, charge coupled device, or magnetic bubble memory, and a random access memory. Mass memories contain a plurality of loops or tracks which are each composed of a closed sequence of memory cells. Each memory cell contains one bit of binary data. The cells of each loop circulate one at a time past a corresponding read/write device which either outputs data from the memory cells or inputs data to the memory cells. Each cell is identified by an address that specifies its place in the loop sequence starting from a reference cell. A loop position counter is set at zero when the reference cell is at the read/write device and the loop counter is advanced by one count each time a new memory cell is moved into operative relationship with the read/write device. The loop position counter contains the same number of steps in its counting cycle as there are memory cells in the memory loop. Thus, the number that is in the loop position counter at any time is the address of the memory cell that is in operative relationship with the read/write device at the same time.
In the past, to read data from the mass memory or write data into the mass memory, it was necessary to specify the desired loop and cell number (or numbers) and then to wait until the specified cell or cells moved into operative relationship with the read/write device. The time delay from the moment that the desired loop and cell number are specified to the time when the desired cell moves into operative relationship with the read/write device is called the latency time. The latency time for any given memory cell can range from zero, if the desired cell is in operative relation with the read/write device immediately after the desired cell is specified, to the time that it takes a cell to circulate completely around the loop.
The principle object of this invention is to provide a mass memory access method and apparatus which completly eliminates latency time and makes it possible to transfer digital data into and out of a mass memory without waiting for the desired cell to circulate into operative relationship the read/write device.